It's older than his book, but check out: Raymond Van Dam, "The Many Conversions of the Emperor Constantine," in Conversion in Late Antiquity and the Early Middle Ages: Seeing and Believing, edited by Kenneth Mills and Anthony Grafton (Rochester: University of Rochester Press, 2003), pp. 127–51.

Stark's Rise of Christianity (1996) is still a very useful read. Although Stark essentializes Christianity and neglects source critique, his argument that Constantine's conversion was a response to and not a cause of the rise of Christianity is an important watershed in historiography. The preface and ch. 1 might be sufficient for your needs.

My only MacMullen reference is his "Two Types of Conversion to Early Christianity" (1983), which might be useful but is certainly dated. For more recent bibliography, you might check Averil Cameron, "The Reign of Constantine, A.D. 306–337," in The Cambridge Ancient History, Volume XII: The Crisis of Empire, edited by Alan Bowman, Averil Cameron, and Peter Garnsey (Cambridge University Press, 2005), pp. 90-109. Or see also the most recent edition of Peter Brown's Rise of Western Christendom.